116 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.41. 



account for the radial symmetry of the Pelmatozoa. Accordmg to 

 the hypothesis of A. H. Clark, which will be discussed at length at 

 some future time, the acquisition of radial symmetry is in no wise 

 dependent upon the type of life led by the ancestral forms. Radial 

 symmetry, according to his views, may be evolved equally as well in 

 eleutherozoic as in statozoic stock. It is impossible to completely 

 accept these views, but it does seem probable that absolute fixation 

 throughout the life of the individual for many generations is not 

 essential. 



We may assume that from the very primitive eleutherozoic echino- 

 derm stock there split off a group m which a statozoic condition of 

 life was the prevailing tendency. Within this general stock there 

 probably were strains in which this tendency was more deeply im- 

 pressed than in others. Be tliis as it may it seems likely that in the 

 history of the Pelmatozoa there was a period in wliich a statozoic habit 

 was the prevailing and dommant character. This period lies inter- 

 mediate between that in which freedom is the expression of non- 

 specialization, and that in which an eleutherozoic existence is the 

 expression of a still higher degree of specialization among types of 

 considerable complexity. It is no doubt largely to this intermediate 

 stage that the Pelmatozoa owe the indelible impress of a statozoic 

 existence as evidenced by orthogenetic tendencies made manifest 

 even to the present day. 



Among the Cystidea we find the only eleutherozoic Pelmatozoa 

 that may not be held as having descended from a long line of sta- 

 tozoic ancestors. Even in this class there are but a few of the many- 

 plated types that may be claimed as having evolved from a stock 

 in which fixation had not become effective. These many-plated 

 Cystidea were short-lived and never of commanding importance. 

 With but one exception indeed there seems to have been no line 

 which persisted for any considerable length of time but that was 

 derived from a stock in which a statozoic habit had been deeply 

 ingrained. The exception is that of the Anomalocystidae, where it 

 would seem that an eleutherozoic habit had been early assumed and 

 constantly maintained. It is probable, however, that even in this 

 line descent was had from Cystidea among which fixation at least 

 became irregularly effective. 



Causes for assumjytion of detached existence. — When the general 

 tendency among the Pelmatozoa seems, superficially at least, to be 

 constantly toward the perfection of organs for purposes of attachment, 

 it is difficult to see the reason for such widespread departure from 

 statozoic conditions. The assumption of an eleutherozoic habit may 

 by no means be held as a reversion, but instead is to be considered 

 as a well-defined progressive tendency. The causes of this deviation 

 from the indicated path are not immediately obvious. It is probable 



